Up to now, lead (Pb) based solder materials have been used widely as solder materials. Particularly, Pb—Sn solder materials have been used in which their melting point is changed in the range of from about 183 to about 300° C. by changing the ratio of lead (Pb) to tin (Sn) therein, whereby solder materials having a suitable soldering temperature have been obtained (see for example, Non-Patent Document 1).
However, lead is poisonous and is thus there is a trend towards the abolition of its use, and there have been demands for development of lead-free solder materials.
Among demands for such solder materials, tin (Sn) based solder materials having various compositions, such as an Sn—Ag alloy and an Sn—Cu alloy, have been proposed.
However, tin based solder materials have a melting point of about 220° C., and thus melt at about 220° C., and the tensile strength thereof is reduced significantly around 200° C.
Examples of bodies to be joined with solder materials include power semiconductor modules.
A power semiconductor module is constructed usually by providing a power semiconductor with an insulator so as to insulate the power semiconductor electrically from a current-carrying part. The power semiconductor and the insulator are joined together via a solder or the like.
The power semiconductor module is provided with a radiator plate to efficiently diffuse or temporarily disperse generated heat from a semiconductor element, and the radiator plate and the insulator are joined together via a solder or the like.
Accordingly, the power semiconductor module has been joined generally at two portions; that is, at a portion between the semiconductor element and the insulator and at a portion between the insulator and the radiator plate.
Up until now, the Pb—Sn solder materials containing lead and tin in varying ratios have been used in these two joining portions of the power semiconductor module.
Next-generation power semiconductor elements GaN and SiC have heat resistance at 200° C. or more, a high dielectric breakdown electric field and saturation electron density etc. and can thus deal with a large current at a high operating voltage. Due to the magnitude of this current, the semiconductor element generates heat to a temperature of about 200° C., and so soldered joining portions are also required to be heat-resistant at 200° C. or more.
Harmless solder materials with a melting point of higher than 250° C. are found occasionally only at the research stage, and only a few come into practical use. There are examples where an Au—Sn alloy (having a melting point of 280° C.) has been used for specific applications, but this alloy is a material containing 80% Au and is thus very expensive and difficult to apply to consumer devices.
As one type of harmless joining material having a melting point higher than 250° C., an Ag based brazing material is generally known, but this material has a high melting point of 600° C. or more such that, when it is used in joining by melting at such a temperature, it damages and denatures semiconductor elements, thus making it unusable for the present purpose. In consideration of the process of manufacturing semiconductor modules, the maximum heating temperature usable in joining is about 450° C.
Under these circumstances, zinc based materials have been examined as solder materials having high melting points. For example, solder materials including Ge and Mg added to Zn—Al alloys (see, for example, Patent Document 1), solder materials including Mg and Sn added to Zn—Al alloys (see, for example, Patent Document 2), solder materials including Mg and In added to Zn—Al alloys (see, for example, Patent Document 3), solder materials including Ge, Sn and In added to Zn—Al alloys (see, for example, Patent Document 4), solder materials including Ge and Mg added to Zn—Al alloys (see, for example, Patent Documents 5 to 7), solder materials including Ge and P added to Zn—Al alloys (see, for example, Patent Document 8), and solder materials including Ge, Mg and P added to Zn—Al alloys (see, for example, Patent Document 9) have each been disclosed.
Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 11-288955
Patent Document 2: JP-A No. 11-208487
Patent Document 3: JP-A No. 11-172354
Patent Document 4: JP-A No. 11-172353
Patent Document 5: JP-A No. 11-172352
Patent Document 6: JP-A No. 2000-208533
Patent Document 7: JP-A No. 2000-61686
Patent Document 8: JP-A No. 2004-358540
Patent Document 9: JP-A No. 2004-358539
Non-Patent Document 1: Yoichiro Baba “Dealing with HV Inverter Quality Maintenance”, Japan Welding Society, National Meeting, Lecture Summary, Chapter 77 (2005-9), Japan Welding Socity